Vacuum pumps are used in the semiconductor manufacturing field, the manufacturing field of flat panel display devices and so on, and many other industrial fields that require depressurization. As the vacuum pump, use is made of, for example, a screw pump. The screw pump is disclosed, for example, in Non-Patent Document 1 as a screw type pump.
Generally, the screw pump comprises a pair of screw rotors having a first screw rotor with a plurality of helical land portions and a plurality of helical groove portions (a male rotor with convex thread ridges) and a second screw rotor with a plurality of helical land portions and a plurality of helical groove portions (a female rotor with concave thread grooves) and adapted to rotate about two axes substantially parallel to each other while meshing with each other, and a casing receiving therein the pair of screw rotors and having an inlet port and a discharge port. Further, a pair of shafts supporting the pair of screw rotors are provided with a pair of bearings and a pair of shaft seal members.
In the conventional pump, ball bearings are generally used as the bearings. Therefore, seal mechanisms such as oil seals or mechanical seals are added between the screws and the ball bearings and further a large amount of gas is introduced to the seal portions. However, since it is not possible to completely prevent leakage of ball bearing oil to the screw (pump) side, in case of being used as a vacuum pump in a processing step (plasma etching, reduced-pressure vapor phase epitaxy) that emits a toxic gas, a corrosive gas, or the like in a depressurized state when, for example, manufacturing semiconductor elements, the gas contacts the ball bearings to corrode the bearings or the oil at the bearing portions flows into the inside of the pump, thereby causing serious failure in the processing step. Further, there has been a technical problem that reaction product is accumulated on the ball bearings to impede smooth operation.
Further, there has been a big technical problem that since the gas introducing amount is large, a huge cost is required for separating and recovering an expensive gas such as Kr or Xe used in the processing step.
Non-Patent Document 1: “Physics Dictionary” compiled by Physics Dictionary Editorial Board, Baifukan, Revised Edition published May 20, 1992, p. 1019